The invention relates generally to integrated circuit fabrication and, in particular, to methods for operating a nanoprober to electrically probe device structures of an integrated circuit.
Specialized electron beam instruments, known as SEM nanoprobers, consist of a scanning electron microscope (SEM) and a set of probes disposed inside the SEM vacuum chamber. Secondary electron images from the SEM are used to position the probes relative to a device structure of an integrated circuit that is to be electrically tested. The probes are used to electrically characterize the device structure. Nanoprobing is capable of directly measuring basic transistor parameters such as threshold voltage (Vt), off state leakage current (Ioff), saturation current (Isat), and junction behavior by means of current/voltage (I/V) curve measurements. Among other applications, these electrical measurements may be useful in performing root cause failure analysis of defective device structures.
The probes and device contacts are real-time imaged with the SEM to land the probes and then to monitor future probe behavior while the electrical measurements are executed. The electron dose, or total charge of electrons per unit area, deposited in the device structure under testing is proportional to the beam current, the exposure time, and the scanned surface area through the magnification. To combat alteration of the device electrical characteristics from electron beam exposure, characteristics of the electron beam, such as accelerating voltage, magnification, and beam current, are minimized. However, these operating characteristics must be maintained above a threshold capable of inducing enough secondary electron emission to permit adequate imaging of the probes and the device structure under testing.
As the probes land on the device contacts, the probes will typically settle in the Z-direction, which may cause the sub-micron probe tips to shift in a lateral (X-Y) direction relative to the contacts. The lateral shifting can bend the probe, which may result in damage to the probe tip or to the device under testing. The probe may also slide off the contact, which results in a loss of electrical continuity. To monitor for lateral shifting, the instrument user monitors each probe with secondary electron imaging. If needed, the instrument user adjusts the pressure of the probe tips while landing on the contacts and often during the subsequent electrical measurement to combat building pressure. The instrument user can note pressure building on a probe by observing increases in the arc or bend of the probe shank or actual lateral motion of the probe tip in the secondary electron image and take corrective actions as needed.
The primary electron beam of the SEM must be scanned across the sample at a slow speed and with a high magnification to permit the instrument user to detect lateral probe movement in the secondary electron image. Unfortunately, slow speed, high magnification imaging with the primary electron beam increases the electron dose absorbed by the device structure, which can unfortunately alter the electrical characteristics. The accumulating electron beam dose also causes sample charging, which reduces the resolution and quality of the secondary electron image.
One conventional solution for alleviating the problems associated with accumulating electron beam dose is to fully blank the primary electron beam after touchdown of the probes on the contacts and during the electrical measurement. In this drastic approach, one instance of a targeted device structure is probed with slow speed, high magnification imaging and without regard to electron dose alterations in electrical characteristics. Then, retaining the same spatial arrangement for the probes and with the beam blanked, the sample stage is precisely stepped to other instances of the device structure and the probes are blindly lowered to establish contacts without imaging. The electrical measurement is then executed in the absence of any imaging with the SEM.
Full beam blanking reduces the beam exposure but completely suspends secondary electron imaging. If sudden movement occurs as a result of building pressure, the probe may deform and bend. Without real time imaging, bent probes cannot be detected until after the conclusion of the probing session when imaging is re-initiated. By then, the probe may be irreversibly damaged or shorted probes may have destroyed the device structure under testing.
As shrinking technology nodes lead to thinner films and smaller device features, mere optimization of the beam characteristics of the SEM used by the nanoprober may prove inadequate. Specifically, imaging with conventional optimized conditions may not be adequate to accurately land the probes on the contacts and, at the same time, to ensure that the electrical characteristics of the device structure under testing do not shift as a result of electron beam exposure. Conventional SEM nanoprobers have reached equipment limits of beam optics as far as the ability to further reduce the accelerating voltage and beam current of the primary electron beam. In addition, full beam blanking is not a viable solution because the smaller probe tips needed at smaller technology nodes are even more prone to lateral movement.
What is needed, therefore, are improved methods for reducing the electron beam irradiation of sensitive regions of a device structure, especially those device structures fabricated with smaller technology nodes, during a probing session in an SEM nanoprober.